The available space under the hood of recently manufactured automobiles has been continually decreasing due to both the added accessories and required safety and pollution devices.
In automobiles equipped with one type of power brakes, a servomotor is required to provide an output force needed to operate the master cylinder. These servomotors usually comprise a cylinder or shell having a front section and a rear section joined together by a twist lock type connection, similar to that shown in U.S. Pat. No. 3,109,346, having a reciprocable piston located therein which supplies an output force. This type of twist lock arrangement extends past the periphery of the shell resulting in additional needed installation space. Attempts have been made to develop a connection between the front and rear shell by the use of interlocking lugs which permit the diameter of the piston to be only negligibly smaller than the shells. In such a servomotor, the diaphragm of the piston has an external marginal portion which is approximately equal to the internal diameter of the front shell. The rear shell engages the marginal portion and presses the same against the internal diameter. However, in manufacturing the shells, eccentricity develops and if the shells are not matched, a vacuum leak can occur. In attempting to compensate for these eccentricities, a greater space between the lugs on the front shell and projections on the rear shell was provided to allow a small amount of radial rotation between these members to uniformly seat the marginal portion. However, with this arrangement, the edge of the marginal portion of the diaphragm is exposed to contaminants which can deteriorate the diaphragm over an extended period of time, again creating a condition where vacuum leak can occur.
In my U.S. Pat. No. 3,768,379, a servomotor is constructed by a disc which encloses an annular front shell by radially confining a bead on a diaphragm against the annular shell while tabs on the annular shell are bent to hold the bead against an annular groove or step to axially seal a front chamber from a rear chamber. However, under some manufacturing conditions it is possible for the disc to cut the bead and produce a potential leak between the front chamber and the rear chamber.